1. Field of the Invention
This invention relates to a method and apparatus for the micro-analysis of the surface of a sample, and particularly to secondary ion mass spectrometry.
2. Description of the Prior Art
In secondary ion mass spectrometry (SIMS), a sample is bombarded by primary ions causing the emission of secondary ions characteristic of the composition of the surface layers of the sample. More generally, secondary ions may be caused to be released from a surface by other forms of primary radiation which may comprise laser radiation, electrons or neutral atoms. After release, the secondary ions are collected and then analysed by the techniques of mass spectrometry. For example, a SIMS instrument may comprise a double focusing mass spectrometer having an electrostatic energy-focusing analyser and a magnetic sector mass analyser. Alternatively a SIMS instrument may comprise a time-of-flight analyser. Two-dimensional images of the surface of a sample may be obtained by direct imaging of an area on a surface, or by scanning a finely focused probe across a surface. Techniques and apparatus for SIMS have been reviewed by G. Slodzian in Advances in Electronics and Electron Physics, supplement 13B, pages 1 to 44, 1980 and by H. Liebl in Scanning, volume 3, pages 79 to 89, 1980.
In general, the secondary ions are collected from the surface by an extraction field and they pass, in some instances via transfer optics, to the mass analyser. The secondary ions are thus accelerated in the extraction field so that they arrive at the mass analyser with a velocity suitable for the mass analyser to function. For example, a magnetic sector mass spectrometer would require the ions to be accelerated through several kV. It is usually convenient to maintain the entrance to the mass analyser substantially at earth potential and to maintain the sample at an electric potential of a polarity to repel the ions of interest and of a strength which will accelerate the secondary ions to the velocity required for analysis in the mass analyser.
In conventional instruments, as illustrated by H. Liebl (op cit), an earthed extraction electrode is positioned close to the surface of the sample to establish an extraction field. There is an advantage, particularly for direct imaging instruments, in having a high extraction field strength because, as discussed for example by G. Slodzian (op cit), the minimum distance that can be resolved between two points on a surface is inversely proportional to the extraction field strength. An alternative arrangement, intended to increase the extraction field near to the surface and so improve spatial resolution, has been described by H. Liebl in Optik, volume 53, number 1, 1979, pages 69 to 72. In that apparatus the extraction field is increased by applying an attractive potential to the extraction electrode. So, to extract positive ions for example, the surface is maintained at approximately +5 kV and the extraction electrode is maintained at approximately -15 kV: the ions are thereby accelerated in a high field away from the surface and are subsequently decelerated in travelling to an earthed electrode. The geometry of H. Liebl's design is similar to that of the Bruche-Johansson lens, commonly used in electron microscopes, and described by P. Grivet in Electron Optics, Pergamon Press, Oxford, 1972. In electron microscopes the potential of the electrode nearest to the surface, known as the Wehnelt of the Bruche-Johansson lens, is adjusted to facilitate fine focusing of the image.
It is also known, as in the non-imaging mass spectrometer described in United Kingdom Patent No. 1,185,203, to provide an extra electrode disposed between the sample and accelerating electrodes. In that instrument the extra electrode is biased to establish potential barrier through which the secondary ions must pass before entering the mass spectrometer. The purpose of the extra electrode is to reduce the flux of ions originating in the residual gas which would otherwise interfere with the spectrum of the ions from the surface.
When SIMS is used to study a sample of electrically insulating material, electrical charge may accumulate on the surface near to the site of incidence of the primary ion beam. The accumulated charge may repel the incident beam, and reduce or even eliminate the secondary ion emission. The mechanism of the charging process depends upon the polarity of the incident ions, the direction of the extraction field, and the nature of the sample. Contributions to the balance of charge at the surface come from the primary ion beam, secondary ions and secondary electrons, though in general the secondary ion yield is low and is therefore not a major contribution to the charge balance. The processes of surface charging have been discussed in detail by H. Werner and A. E. Morgan in the Journal of Applied Physics, volume 47, pages 1232 to 1242, 1976.
One known method of alleviating the accumulation of surface charge is to deposit a conductive grid onto the surface, though this may introduce contaminants from the material of the grid into the spectra. Werner and Morgan have described how a diaphragm in contact with the surface and having an aperture larger than the extraction area of secondary ions can also reduce surface charge. Moreover, specifically to alleviate negative charge which accumulates during bombardment by negative primary ions, such a diaphragm may be placed close to but not in contact with the surface and biased slightly positive with respect to the surface; the charge is then reduced because secondary electrons are attracted to the diaphragm. However, like a grid, a diaphragm may inhibit the detection of secondary ions from the sample if the primary ions strike the conductive material and if that material has a significant sputter yield: Werner and Morgan acknowledge that the diaphragm does restrict lateral imaging of the sample.
In SIMS, if positive primary ions are used to analyse insulating samples a positive charge will tend to accumulate on the surface. A technique commonly employed to alleviate this is to apply a primary electron 'flood' beam to the surface concurrently with the primary ion current. For this to neutralise the surface charge there must be a balance between the significant currents, which generally are: the primary ion, primary electron and secondary electron currents. Clearly, if the primary electron flood current is too great, the surface can become negatively charged near to the site of beam impact. When the sample is biased negative with respect to the earthed extraction electrode, in order to extract negative secondary ions, an electron flood beam initially of high energy, will be retarded and will reach the surface with low energy. G. Slodzian et al, in Microbeam Analysis 1986, published by the San Francisco Press Inc, page 78, has described apparatus in which a high-energy electron beam is directed perpendicularly towards a surface. Electrons in the incoming beam are slowed down as they approach the surface, and subsequently neutralise the positive surface charge. Alternatively, when the sample is biased positive in order to extract positive secondary ions, a primary electron flood beam can be directed and accelerated towards the surface. However, if the flood current is too large the surface may charge negatively and inhibit the release of positive secondary ions. Excessive negative charging may be alleviated by the release of secondary electrons, though when the sample is at a positive potential these will be attracted back to the surface by the extraction field. The balance between currents in this case has been discussed by Werner and Morgan who concluded that secondary electron emission could effectively reduce the local negative charging due to excessive flood current only if the sample were to charge up to the extraction electrode potential, but this Would drastically reduce the secondary ion current.
In apparatus for SIMS employing negatively charged primary ions an insulating sample may charge negatively, with consequent degradation of performance, and even with a neutral primary beam there may be some charging of the surface resulting from the loss of secondary particles, notably secondary electrons.
Further problems in SIMS arise in the study of samples with irregular surfaces. Surface roughness causes variation in the angles at which secondary particles leave the surface and corresponding variation in the intensity of the detected secondary signal. Bedrich et al, in the Springer Series In Chemical Physics, volume 19, SIMS III, pages 81 to 87 show the variation in intensity of a two-dimensional surface image caused by a roughness of up to 5 .mu.m.
In summary, while SIMS is an established and important technique of surface analysis there are, nevertheless, aspects which could usefully be improved, particularly in the analysis of insulating samples, or samples with irregular surfaces.